Method and apparatus using shaped field of repositionable magnet to guide implant

ABSTRACT

Methods and apparatuses for displaying and using a shaped field of a repositionable magnet to move, guide, and/or steer a magnetic seed or catheter in living tissue for medicinal purposes. A moveable magnet assembly and a portion of a patient&#39;s body undergoing magnetically-aided surgery are both provided with fiducial markers. The portion of the patient&#39;s body is fixed in a location in which the fiducial markers are sensed and located by a set of localizers. The positions of the fiducial markers are determined by a processor, which operates on a stored representation of the magnetic field of a magnet in the magnet assembly to provide a display of the present magnetic field of the magnet. This display may be superimposed over an MRI, X-ray or CAT image during surgery. The repositionable magnet can be an electromagnet. In some embodiments, a computer calculates orientations and currents for an external electromagnet to move the implanted magnetic object in the patient&#39;s body through a desired path. The display can provide real-time imaging of the implanted magnetic object in comparison to the desired path as the object is moved.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a division of application Ser. No. 09/189,646, filedNov. 10, 1998, now U.S. Pat. No. 6,212,419, and which claims the benefitof U.S. Provisional Application No. 60/065,103, filed Nov. 12, 1997,entitled “Method and Apparatus Using Shaped Field of RepositionableMagnet to Guide Implant,” and which is herein incorporated by referencein its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to devices and methods for using a magnetic fieldto guide a surgical implant, and more specifically to devices andmethods for using the near field and transition field of arepositionable magnet to move, guide, and/or steer a magnetic seed,catheter or other magnetic delivery vehicle (MDV) for therapeutic orsurgical purposes.

2. Description of Related Art

In the field of surgery, there exists a need to control the orientation,forces, and/or motion of internally implanted devices. One method thathas been used to control such implanted devices is the application of amagnetic field from an external magnet. In this method, the magneticfield acts upon the implanted device, which itself comprises magneticmaterial, which may be in the form of a permanent magnet. In accordancewith prior art practice, a physician surgically implants the devicecomprising magnetic material and then guides the position of theimplanted device by moving an external permanent magnet and observingthe resultant movement directly with an X-ray fluoroscope. Examples ofthe prior art may be found in a review article by Gillies et al.,“Magnetic Manipulation Instrumentation for Medical Physics Research,”Rev. Sci. Instrum. 65, 533 (1994), and references cited therein. Seealso McNeil et al., “Functional Design Features and Initial PerformanceCharacteristics of a Magnetic-Implant Guidance System for StereotacticNeurosurgery,” IEEE Trans. Biomed Engrg., 42, 793 (1995); Tillander,“Magnetic Guidance of a Catheter with Articulated Steel Tip,” ActaRadiologa 35, 62 (1951); Frei et al, “The POD (Para-Operational Device)and its Applications,” Med. Res. Eng. 5,11 (1966); U.S. Pat. No.3,358,676 to Frei et al., issued Dec. 19, 1967, entitled “MagneticPropulsion of Diagnostic or Therapeutic Elements Through the Body Ductsof Animal or Human Patients”; Hilal et al., “Magnetically Guided Devicesfor Vascular Exploration and Treatment,” Radiology 113, 529 (1974);Yodh, et al., “A New Magnet System for Intravascular Navigation,” Med. &Biol. Engrg., 6, 143 (1968); Montgomery et al., “Superconducting MagnetSystem for Intravascular Navigation,” Jour. Appl. Phys. 40, 2129 (1969);U.S. Pat. No. 3,674,014 to Tillander, issued Jul. 4, 1972, entitled“Magnetically Guidable Catheter-Tip and Method”; and U.S. Pat. No.3,794,041 to Frei et al., issued Feb. 26, 1974, entitled“Gastrointestinal Catheter.” The full content of each of the citeddocuments are herein incorporated by reference in their entirety.

Unfortunately, the above-described technique requires the physician toreact to the movement of the implanted device after the fact. There isno precise correlation of the imaging system with the medical magneticmanipulation, and no way to apply fields and/or force gradientsprecisely in needed directions. With hand-held magnets, the onlyfeedback the surgeon could have was his observation of motion of amagnetic implant by x-ray or ultrasonic imaging in response to hismovement of the magnet. The field producing magnet, so guided withoutdirect visual display of the field lines, is controlled by theoperator's estimate of the field direction and magnitude at a particularlocation of the implant. Since many combinations are possible, thisessentially “blind operation” is bound to result in a somewhat randomposition and angulation as related to the needed field line directionand magnitude to provide guidance and/or pulling force. In difficultinterference situations, it is difficult without such imaging guidanceto provide even a reasonable guess as to a correct direction for themagnet axis to obtain field alignment with the intended path. The largeelectromagnet of Yodh et al. is one attempt to minimize the “blindness”of the approach just described, but the Yodh et al. approach stillrelies on operator judgment and vision, and is subject to such error.While multiple coil arrangements such as the magnetic stereotaxis system(MSS) described in McNeil et al. can be used to provide such guidance,it is difficult in such systems to provide a combined guiding force andforce-applying field gradient in the same desired direction.

Clearly, both operation time and risk to a patient could be reduced ifan apparatus and method were available to more accurately and rapidlyguide or move a magnetic surgical implant. This device and method caneither provide feedback to the physician guiding the implant so that thephysician could predict the movement of the implanted device rather thanreact to it after the fact, or it can be used more automatically withcomputer-controlled motion along a physician-selected planned path. Itwould also be advantageous if simpler hardware and software could beused to locate the external magnet and provide more effective fieldsolutions. The moveable magnet location should take into account anexclusion volume around the patient in which the magnet may not belocated. In the case of neurosurgery, for example, the magnet cannot belocated too closely to the patient's head, nor in the path of imagingX-rays.

SUMMARY OF THE INVENTION

It is thus an object of the invention to provide a rapid interactivedisplay of the aligning torque and magnetic pulling directions of amagnet acting on a volume.

It is a further object of the invention to provide a physician withdevices and methods that facilitate the prediction of the movement of amagnet implant in response to an externally applied magnetic field.

It is yet another object of the invention to provide devices and methodsthat facilitate rapid and appropriate adjustment of the position of anexternal magnet to steer a magnet internal to the body of a patient.

It is still another object of the invention to provide a moveablefield-producing magnet that can be located and angled so as to provideflexibility in avoiding interference with imaging systems which maychange between and during various surgical procedures.

It is another object of the invention to provide means whereby aphysician can use voice control or other non-tactile control to governpath choice at arterial branches or lumenal branches so that both handsare available for other needs, in intravascular navigation applications.

It is yet a further object of the invention to provide an externalmagnet adapted to provide a magnetic field of sufficient strength andappropriate angular spread to provide flexibility in positioning andorienting the external magnet, even while respecting exclusion volumesaround a patient's body.

These and other objects are achieved by the inventive methods andapparatuses to guide an implant disclosed herein. The invention providesrapid interactive display of the aligning torque and magnetic pullingdirections of a permanent, a superconducting, or a resistive wire magnetacting on a volume, which volume may include a portion of a patient'sbody. The invention allows a hand-held, hand-positionable orservo-controlled external magnet to be moved external to the volume,while the resultant magnetic forces are displayed in real-time(essentially instantaneously, or at least as rapidly as is needed foreffective surgical control) on a computer screen. In this way, aphysician can rapidly adjust the position of an external magnet to steera magnet internal to the body.

By placing a set of fixed fiducial marks on the magnet and using adevice that can localize these marks in three dimensions, the positionof the magnet can be associated with the treatment volume. The volume'sposition can be associated with the magnet by placing fiducial marks onthe volume and “registering” these marks with the localizer, or byputting the volume in a standard place relative to the localizer. Asecond method of localizing the magnet includes putting magnetic sensorsat appropriate fiducial points.

The magnetic field of any magnet can be simply measured and suitablymapped in three dimensions. At run time a pre-measured map can besuperimposed on the imaging volume using the registration established bythe fiducial marks.

Assuming that the volume has a set of images associated with it andfiducial marks that can be registered in the imaging volume, themagnetic field and force can be displayed and updated as the magnet ismoved relative to the imaging volume. The field can be displayed at anypoint in the image set, and updated as the magnet is moved. Commercialultrasonic and infrared localizers exist at present that give greataccuracy (on the order of a millimeter) to this technique. One suchlocalizer is described in U.S. Pat. No. 5,383,454, issued Jan. 24, 1995to Richard D. Bucholz, which is hereby incorporated in its entirety byreference.

In one embodiment, an apparatus in accordance with the inventioncomprises a moveable magnet assembly having a plurality of fiducialmarks thereon; a localizer comprising a plurality of imaging sensorsconfigured to sense a position in three-dimensional space of thefiducial marks to thereby provide an indication of a location of themoveable magnet; and a processor including memory having stored thereina pre-measured representation of a magnetic field generated by themagnet assembly and a display configured to present a graphicalrepresentation of the magnetic field, the processor being configured tobe responsive to the indication of the location of the moveable magnetprovided by the localizer so that the display provides an indication ofthe magnetic field within a selected volume of space resulting from themagnet assembly when the magnet is at a position indicated by thelocalizer.

In accordance with another embodiment of the invention, a method forproviding a rapid interactive display of the aligning torque andmagnetic pulling directions of a magnet comprises the steps of providinga plurality of fiducial marks on the magnet assembly; sensing a locationand orientation of the fiducial marks; computing a magnetic field in aselected volume of space produced by the magnet assembly when themagnetic assembly is in the sensed location and orientation; anddisplaying a representation of the magnetic field in the selected volumeof space. This inventive method embodiment may further comprise thesteps of placing a magnetic seed in a tissue of a living body; providingfiducial marks on an external surface of the living body proximate thetissue; sensing a location and orientation of the living body; andselecting the selected volume of space to include the tissue of theliving body.

In another variation of the invention, the field lines need not bedisplayed. Instead, a vascular or other lumenal path displayed on thescreen is surgeon identified as being the path of choice. The computeris then able to “know” in the treatment volume the direction needed forthe guiding field at each point along the path. It can then provideorientation and positioning of the external, field-generating magnet asneeded for navigation along the path. At arterial or other lumenalbranch points, the surgeon can further assist the delicate navigationalneeds by voice commands understandable by the computer.

In other variations of the inventive methods and apparatuses, means areprovided for locating and angling the external field producing magnetwhile avoiding interference with the imaging means, which may include anexclusion volume about a patient's body. In yet another variation, apermanent, superconducting, or resistive wire external field-generatingmagnet provides a “shaped” field, i.e., the near (and/or transition)field of the magnet is produced by a specially shaped coil and/or coreto provide magnetic fields of sufficient strength and appropriateangular spread to provide additional flexibility in positioning themagnet during surgery. In the case of a superconducting magnet, thewinding shape is adjusted. For a resistive wire magnet, the windings andcore of the magnet may both be adjusted to accomplish the shaping.

These and other embodiments of the invention may be fully understood bythose skilled in the art by reference to the drawings and to thedetailed explanation that follows.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram representation illustrating an embodiment of adevice in accordance with the invention;

FIG. 2 is a block diagram showing how coordinate systems relative to amagnet, a set of localizers (cameras), and an image on a display arerelated;

FIG. 3 is a block diagram illustrating another embodiment of a device inaccordance with the invention;

FIG. 4A is a side plan view of an embodiment of an electromagnet havinga shaped magnetic field in accordance with the invention;

FIG. 4B is a side plan view of the electromagnet of FIG. 4A showing how“magnetic lensing” is accomplished;

FIG. 4C is an illustration of a representative field of a single coilmagnet showing off-axis field lines;

FIG. 5A is an illustration of a typical exclusion zone for neurosurgery;and

FIG. 5B is another illustration of the exclusion zone of FIG. 5A showingboth a typical cumulative mechanical zone of exclusion and a typicalmaximal operating volume for a magnetic catheter in a brain.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

As used herein in this description and in the claims that follow, unlessotherwise explicitly qualified or made clear by the context in which itappears, the term “magnet,” when referring to a magnet external to apatient, is intended to include both single magnets (of either thepermanent or electromagnetic variety) and assemblies of a plurality ofmagnets in which the individual magnets of the assembly are used inconjunction with one another to produce an effective magnetic field.

FIG. 1 is a schematic representation of an embodiment of a device 10 inaccordance with the invention. A moveable magnet assembly 12 including amagnet 14 is provided. Magnet assembly 12 may be a gantry supporting theweight of magnet 14. Magnet 14 may be either a strong permanent magnet,a superconducting electromagnet, or a resistive wire electromagnet,although a strong permanent magnet may require additional articulationto compensate for its lack of current control and magnitude. Motion ofthe magnet assembly 12 may be automatically controlled, such as with acomputer- or servo-controlled robotic arm 16 or other equivalentmechanical manipulation device to provide the needed orientation,location, and consequent coil current required to align its magneticfield with the desired motion of a magnetic object 30 to be guided.However, automatic control is not required by the invention. Thus,magnet assembly 12 may simply comprise a permanent magnet 14 at the endof a wand 16 (rather than a robotic arm) that a surgeon manipulates byhand. If automatic control or gantry support is provided, however, theapparatus can smoothly manage relatively large electromagnetic coils 14,and can therefore apply strong magnetic fields, so as to provideguidance or force deeper into the body 26.

Device 10 further comprises a plurality of localizers 20, which may bemounted on a localizer assembly 18. The plurality of localizers 20 eachcomprise a camera-like sensor (such as a commercial infrared, optical,or ultrasonic sensor) that senses fiducial marks 22 on magnet assembly12. A general discussion of localizer technology of a type that may beemployed in the present invention will be found in previously-mentionedU.S. Pat. No. 5,383,454. Fiducial marks 22 on magnet assembly 12 may beany marker that can be located by the plurality of localizers 20, andmay preferably comprise lighted LEDs (light emitting diodes).Optionally, additional fiducial marks 24 may be applied to a patient'sbody 26 or part thereof (in the illustrated case, a patient's head).Fiducial marks 24 may either be screw-in or stick-on markers. Screw-inmarkers are preferable for the greater accuracy that may be obtained.Alternately, anatomical landmarks can be used, especially if thelocation of the volume of interest 28 is known relative to localizers 20and a CAT, X-ray or MRI medical imaging device is used in conjunctionwith the localizers 20 (as is normally contemplated). However, the useof anatomical landmarks may result in less accurate display of themagnetic field of magnet 14 relative to the patient's body 26 and anymagnetic seed 30 or other surgical device embedded in the patient'sbody. (The term “seed” is intended to encompass all types of magneticprobe masses or other magnetically guided or pulled implanted surgical,diagnostic, or therapeutic objects in the body.)

While mounting of localizers 20 on a localizer assembly 18 is desirable,any alternate means of positioning and directing the localizers at aspecified volume of space in which fiducial marks 22 may be located whenmagnet assembly 12 is in use may be used. It is only necessary thatfiducial marks 22 be positioned in such a way that the position andorientation of the magnet 14 in three-dimensional space can be sensed bylocalizer 20 relative to a specified volume of interest 28. In practice,this volume of interest 28 would include the location within which theportion of the patient's body 26 in which a seed 30 or magnetic guidancetip (such as at the end of a catheter) is embedded when an operation isperformed, and is preferably within a magnetic resonance imaging (MRI)or other area sensed by such means as would normally be employed duringmagnetic surgery. Seed 30 or magnetic guidance tip may comprise apermanent magnet, or an induced magnet of a permeable material. Forsimplicity, either type of material may be referred to as a “magnet” forpurposes of describing the implant. Fiducial marks 24 must either belocated by tracking them with a lighted probe (such as a probe withLEDs) or by touching a calibrated portion of the magnet to each in turn.A sufficient number of localizers 20 and fiducial marks 22 are providedto enable a processor 32 receiving signals from localizers 20 toaccurately determine the position and orientation of magnet 14. Iffiducial marks 24 are provided on the patient's body 26 for the purposeof locating the position and orientation of the patient's body 26relative to magnet 14, a sufficient number of these such marks 24 arealso provided to enable the processor 32 to make a determination of thelocation and orientation of the patient's body 26. Any fiducial marks 24provided on patient's body 26 would be of a type that would also appearin a CAT, X-ray, or MRI scan in order for these images to be registered.

Localizers 20, whether optical, infrared, ultrasonic, magnetic, orotherwise, detect the position of the markers 22 on magnet assembly 12and provide a signal or signals to processor 32. It will be understoodthat the nature of the communications link between localizers 20 andprocessor 32 is not critical, nor is it critical as to whether aseparate link is provided between each localizer 20 and processor 32 orwhether data from all the localizers 20 is combined and sent through asingle link. Processor 32 is also provided with memory 34 in which isprovided data representative of the magnetic field in three dimensionsproduced by magnet 14, and memory 36 in which is provided a storedprogram containing instructions for processor 32. (Memory 34 and memory36 in FIG. 1 may be any suitable, conventional form of memory and maycomprise different addresses or locations in the same physical memoryelement.) The representation for the magnetic field may be a set ofmeasured data or a set of parameters for magnet 14 from which themagnetic field can be calculated. If magnet 14 can be one of severalmagnets of varying strength or size, data or parameters for severalrepresentations of magnetic field, each representing a different magnet,may be stored in memory 34. If magnet 14 is an electromagnet rather thana permanent magnet, the data or parameters in memory 34 may be scaled inaccordance with an amount of current flowing through magnet 14. Datarepresenting this amount of current would be made available as an input(not shown) to processor 32, preferably as a signal obtained from apower supply of magnet 14 or an ammeter (neither of which is shown).

In some modes of operation, the processor 32 will, in addition to theabove tasks, calculate the current needed by the electromagnet 14 tooptimally provide a field at the location of seed 30. The position,orientation, and current of magnet 14 will be optimized simultaneouslyby processor 32 so as to yield the needed navigation with a minimum ofmagnet motion. In such modes, the processor will send a signal (notshown) to the electromagnet power supply.

The program provided in memory 36 instructs processor 32 to calculate,from the localizer inputs that result from fiducial marks 22, thelocation and orientation of magnet 14 relative to the volume of interest28. From this location and orientation, the program further instructsprocessor 32 to provide a display representative of the resultingmagnetic field within the volume of interest 28. In “manual mode,” i.e.,the mode in which a surgeon navigates visually rather than by directingoverall computer navigation choices, this display would be shown on thesurgeon's display 40. Otherwise, a second engineering display (notshown) would present these features. Preferably, the location offiducial marks 24 on the patient's body 26 are also displayed. Therepresentative display of the magnetic field is preferably graphical,and is presented in a fashion that is useful to a surgeon during anoperation. For example, a two-dimensional view in a plane including seed30, or a rotatable three-dimensional perspective view may be provided. Aselection of one or more different types of views may be provided, basedupon operator input, such as through a keyboard, a mouse, a joystick, orother input device or devices 42 communicating with processor 32. Theinput device or devices 42 may also be used in a manner described belowto initialize the necessary coordinate system transformations to beperformed by processor 32.

Data received from localizers 20, and the processing by processor 32 topresent a graphical representation on display 40 of the magnetic fieldproduced by magnet 14 must be fast enough to provide “real-time”feedback for a surgeon; i.e., the feedback must be rapid enough to allowdecisions to be made during a surgical procedure involving the movementof the implanted magnetic device 30. The method of Procrustes is used tocompute the 4×4 rigid body transformation between coordinates in theimaging system and coordinates in the localizer system. Thereafter, the4×4 matrix may be applied to transform a pre-stored representation of amagnetic field into a magnetic field having the position and orientationsensed by localizers 20 using standard programming techniques on apresently-available Intel PENTIUM®-based processor (such as a typicalPC), or a Silicon Graphics workstation, with the transformation beingaccomplished in sufficient time to provide a display that is updatedrapidly enough for surgical purposes.

Preferably, at least three, or even more preferably, four fiducial marks22 are provided on magnet assembly 12, and the same number of fiducialmarks 24 on the patient's body 26. Also preferably, display 40 may alsoprovide a display of a simultaneously obtained CAT, X-ray, or MRI image.The CAT, X-ray, or MRI image can be superimposed upon the display of themagnetic field. If the fiducial markers 24 on the patient's body 26 areof a type that shows up on the CAT, X-ray, or MRI image (e.g., metalscrew-in markers), then the magnetic field display and the CAT, X-ray,or MRI image may be aligned or registered either automatically byprocessor 32 or manually by the surgeon. Alternately, or in addition tothe CAT, X-ray, or MRI imaging device, a magnetic locating device suchas that described in U.S. Pat. No. 5,558,091, issued Sep. 24, 1996 toAcker et al. (which is hereby incorporated by reference in its entirety)may be used for determining and displaying a position and orientation ofimplanted seed 30. It is also possible to use the magnet being used toguide the implant as the source of the magnetic field for determiningthe location of the implant, in accordance with the devices and methodsdisclosed in “Method and Device for Locating Magnetic Implant by SourceField,” a U.S. Patent Application to R. Ritter et al. filed on even dateherewith and hereby incorporated in its entirety by reference.

A typical use of the inventive apparatus would proceed as follows:First, a patient's head (or other body part to undergo magneticallyaided surgical procedure) would be supported in a fixed position inspace relative to localizers 20, with fiducial marks 24 (or anatomicallandmarks) on patient 26 visible to localizers 20. A representation ofthe field of view of localizers 20 then appears on display 40. Fiducialmarks (or anatomical landmarks) are then identified on display 40 bytouching the actual physical locations on patient 26 with magnet 14 orin some other suitable fashion. The image coordinates are related tolocalizer coordinates using the method of Procrustes. Next, the locationof magnet 14 is determined by localization of fiducial markers 22. Oncethe location of the magnet 14 is known, processor 32 then relates thecoordinate system of the stored representation of the magnetic field inmemory 34 to the coordinate system of the images using simple matrixmultiplications. A representation of the magnetic field of magnet 14 inthe coordinate system of display 40 is then presented on display 40,preferably together with a representation of the part of the body ofpatient 26, fiducial markers 22 and 24, and magnetic seed or guidancemagnet 30. As the magnet 14 is moved, positions of the fiducial markers22 change and are detected by localizers 20, and the resulting magnet 14positions determined. In manual mode, display 40 is continuously updatedto show the resulting magnetic field in the coordinate system of display40. In automatic mode, the equivalent information is updated withinprocessor 32. Preferably, a current location of seed or guidance magnet30 is also shown on display 40.

FIG. 2 shows, in block diagram form, the relationship between variouscoordinate systems of interest in the inventive system. The magnetcoordinate system refers to a coordinate system referenced to magnet 14.The representation of the magnetic field of magnet 14 that is stored inmemory 34 is likely to be most conveniently represented relative to thiscoordinate system. The camera coordinate system refers to the coordinatesystem of localizers 20, and may be fixed depending upon theconstruction and positioning of localizer assembly 18. The imagecoordinate system refers to the three-dimensional coordinate system ofthe image of the patient on display 40. Because the positions of theLEDs (or other fiducial markers 22) on magnet assembly 12 are known inadvance relative to the magnet coordinate system, once the actualpositions of the LEDs are known, the magnetic field can be convertedeasily to the other coordinate systems using transformations T_(m),T_(i), or a combination of both transformations (which may be applied ina single step, rather than two separate transformations). Thesetransformations are normally carried out by multiplications involving4×4 transformation matrices. Typically, the LEDs that may be used asfiducial markers 22 may be infrared LEDs that are detected by camerassensitive to infrared radiation and used as localizers 20. Processor 32receives information from localizers 20 from which it can compute theposition of the LEDs (or the localizer array may contain its ownprocessor to do this and supply processor 32 with this informationdirectly). When the patient's body is fixed in position and fiducialmarkers 24 touched by a point known relative to the magnet, thepositions of fiducial markers 24 relative to the LEDs (i.e., fiducialmarkers 22, the positions of which are known in advance) are determined.The identification of the corresponding marks on display 40 identifiesthe corresponding positions in the image coordinate system. T_(i), isthen computed using the method of Procrustes. Finally, as the magnet 14is moved, the positions of the LEDs are changed and detected by thearray of localizers 20, and the transformation T_(m) is determined andcontinuously updated. Concatenations of transformations of T_(m) andT_(i) are continuously performed to update display 40 or the processorcontrol memory.

FIG. 3 is a block diagram of another embodiment of the inventiveapparatus 10′. In this embodiment, a joystick 42′ is used as an operatorinput to processor 32, possibly in conjunction with other standard inputdevices such as a keyboard (not separately shown). More than one magnetassembly 12 may be provided, each with a separate magnet or magnets 14,in which case memory 34 may be provided with a plurality of magneticfield representations, each representing a particular magnet or magnets14 on a magnet assembly 12, or it may be provided with only one magneticfield representation, if the magnetic fields of magnets 14 aresufficiently similar. Magnet assemblies 12 in this case are moved inthis embodiment under control of operator input device 42′ or anothersuitable input device or devices, and if magnets 14 are electromagnets,the current through magnets 14 may also be controlled via operatorinputs. Localizers 20 and processor 32 are configured to locate each ofthe magnets 14, and processor 32 is configured to compute the magneticfields from each of the magnets 14, compute a resultant total magneticfield, and display the representation of the resultant total magneticfield on displays 40 and 40′. (In this embodiment two displays [40 and40′] are provided as an aid to the surgeon. It will be understood thatas many displays as are deemed necessary may be provided.) Possiblydifferent colored LEDs may be provided on the individual magnetassemblies 12 so that their separate locations are more readilydetermined. Preferably, if a plurality of magnet assemblies 12 areprovided, they may be used either independently or in unison, asrequired.

Processor 32 also controls two X-ray tubes 44 (one for each display 40,40′). For each X-ray tube 44, there is one sensor (camera), not shown,the output of which is the signal sent to processor 32 to provide thedisplays. These X-rays provide real-time imaging of patient 26 as wellas a surgical device 46 being steered by the magnetic fields of magnets14. Preferably, the fluoroscopic imaging system is immune to magneticfields, because of the presence of the strong fields from nearby magnet14. Sensors for fluoroscopic imaging systems meeting this requirementinclude charge coupled devices (CCDs), micro channel plates (MCPs), andamorphous silicon technologies. In this case, surgical device 46 is acatheter having a magnetic tip 30. Images provided by X-ray from tubes44 are superimposed on the magnetic field image on displays 40 and 40′to indicate the progress of the surgical procedure to allow a surgeon todetermine how to position magnet assemblies 12 (and possibly controlcurrents in magnets 14) as the operation proceeds. The images providedby X-ray tubes 44 may be provided by sensors (not shown) that provideimage inputs to processor 32. It will be recognized that manycomponents, such as magnets 14, X-ray tubes 44, displays 40 and 40′,input devices 42′, etc., may be provided in such numbers as needed, ifprocessor 32 (which itself may comprise a plurality of processors) isprovided with the appropriate stored program.

In each of the embodiments of the invention, magnet or magnets 14 areorientable and positionable about a patient's body 26. The bestorientation is commensurate with the display, and with avoiding imagingequipment (including X-ray tubes 44 and corresponding sensors) and partsof body 26 itself, which in prior art methods would interfere withadequate positioning and orientation of a magnet. Often, in commonusage, a magnet which pulls or aligns an object does so along its axis,but the inventive apparatuses and methods disclosed herein do notrequire this limitation. These apparatuses and methods provideoptimization of the magnetic field strength and minimization of currentneeded for energization of external magnet 14 (or the plurality ofexternal magnets 14, if more than one is provided).

Several versions of the control of the manipulation can be employed. Ina form employing the least amount of automatic control, screens 40 and40′ display magnetic field lines, either in two (preferably orthogonal)projections, or in a three dimensional representation that can berotated or otherwise manipulated on a screen. A surgeon-operatormanipulates magnet 14 (such as by moving magnet assembly 12), which inturn moves the magnetic field line display on the display screen orscreens (such as screens 40 and 40′), so that the field direction at thelocation of the implant is made to agree with a desired and displayeddirection of motion of the implant. The ability to accomplish thisdirectional agreement, even in the presence of restraints set bypotentially interfering imaging components and parts of the body, isgreatly facilitated by the shape of the magnetic field lines.

In moving away from the axis of an electromagnetic coil used for magnet14, magnetic field lines increasingly curve away from the straight lineof the on-axis field of the coil. These magnetic field lines haveazimuthal symmetry about the coil axis, which often adds to the freedomof coil orientation. Thus, locating and orienting the field sourcemagnet 14 so that a curved field line falls on the location of themagnet, makes possible a direction control that would otherwise begeometrically impossible, because of spatial constraints in many cases,if the axis of magnet 14 had to align with the motion direction of seed30.

If attainment of a specific field strength is required by the controlprogram, the use of current control (and/or closeness of magnet 14)provides an accommodation to the use of the curved field line. In atypical coil, the field strength of the magnetic field at a givendistance from the coil is reduced as an implant 30 is moved off axis andto a location of more curved field lines. For a purely manual version ofcontrol, the position of and/or current in the magnet 14 may be adjustedby the operator in response to marked regions of the displayed lines,which may be marked in distinctive colors. Distinctive colors or othersuitable markings may be displayed to indicate avoidance regions, i.e.,regions so close to a seed 30 or other implant that an undesiredmagnitude of field or gradient is present at the seed or implant. Byavoiding positioning of magnets 14 in the avoidance region or regions,means other than the imposition of a magnetic force or torque on theimplant (such as mechanical motion of a catheter to which the magneticseed 30 is attached at an end) may be used for moving it, if desired.Segmentally colored lines on display 40 and/or 40′ may also be used tohelp guide an operator in his manual manipulation of a magnet or coil byfacilitating the choice of more advantageous position and anglecombinations for magnet 14 for applying a specified magnetic fieldmagnitude and direction at the location of seed or implant 30.

In use, the inventive apparatus causes the magnetic seed or implant 30to align with the magnetic field direction at its location in the body26. In addition, if a pulling force is required, the direction of thegradient on the aligned implant is also along the field line (in anantiparallel direction). If seed 30 is a magnetically permeable implantor an appropriately oriented permanent magnet implant, the implant 30 isnaturally guided along the field line and pulled along it, if desired.

By “shaping” of a magnetic field, it is meant that magnetic field linesof either the near field or the transition field of the magnet, or both,are given a known or readily determinable shape useful for manipulatingmagnetic seed 30, particularly in positions where it would be difficultto apply an on-axis field of proper orientation and magnitude due to anexclusion region, or where it is desirable to guide or pull the magneticseed 30 along a curved path corresponding to a field line of the shapedmagnetic field. The “near field” of a magnet is defined as the field ofthe magnet that is close enough to the source (which may be anelectromagnetic winding) such that it does not correspond to asufficiently close approximation of the standard dipole field shape forsurgical guidance purposes. The near field of a finite source differsfrom a point dipole field significantly, for present purposes, when thefield point is within 3 to 4 times the largest dimensions of the finitesource, such as the greater of the width or length of a coil. A“transition field” is also defined between the near field and the dipolefield, if the magnet is a single coil, or between the near field and thehigher multipole field, in the case of a multicoil system. It iscontemplated that computer or processor 32 (see, e.g., FIG. 1) willpreferably have sufficient data and instructions in software to computethe near and transition fields of a magnet 14 for use for automaticcontrol of magnet 14 or for display on display units 40, 40′ in the caseof manual control. Of course, guidance and pulling of a seed 30 may alsobe accomplished using the far field of magnet 14, but with theadditional information that is preferably available to processor 32, theoff-axis components of the magnetic field of magnet 14 may also becomeuseful for guidance and pulling of seed 30.

One embodiment of a magnet 14 configured to provide a “shaped” magneticfield suitable for use with the inventive apparatus is shown in FIG. 4A.Winding 60, which may be a resistive wire winding, is wound in the shapeof a sphere. With automated winding equipment, a spherical winding maybe readily made by winding more layers in a central portion of thesphere to produce a bulge in the middle section of winding 60. For asuperconductive winding, the special form of winding 60 alone, without acore, is adequate to provide a sufficiently “shaped” magnetic field. Fora resistive winding, a core 62 of permeable magnetic material isprovided to shape the magnetic field in conjunction with the specialform of winding 60. The embodiment illustrated here has a mushroom-likeshape. Of course, other embodiments are possible, including those inwhich magnet 14 is a permanent magnet of predetermined shape.

A permanent or resistive wire magnet 14 can be made to provide “magneticlensing.” A segment of permeable material, such as core 62, may beinserted in winding 60 (or in a hollow permanent magnet) and movedlongitudinally along the front of the permanent magnet or inside thewinding of the resistive wire magnet to change the shape of the fieldlines to accomplish a particular procedure. Different segments ofpermeable material may also be substituted to provide different amountsof shaping of the magnetic field. Spacer rings 64 may be provided, asshown in FIG. 4B, to prevent “suck-in” of the core material 62 thatwould otherwise occur with a permanent magnet 60, or when a winding 60is energized.

Control of the magnetic field strength applied to seed 30 is easilyaccomplished. In a manual embodiment, the operator manipulates theexternal magnet 14 in response to the magnetic field lines displayed onthe display system 40, 40′ so that the magnetic field lines andmagnitude of the magnetic field are configured to provide the correctmagnitude and direction at the location of seed 30 to provide thedesired motion or guidance of seed 30. Control of the magnetic field mayalso be done by software running in control computer 32. In this case,the screen display 40, 40′ provides confirmation of the operation of thesystem to the operator.

In yet another embodiment of the inventive apparatus, display system 40,40′ is less important, and it is not used in the same manner. Instead,the computer 32 is provided with an optimization program in softwarethat processes the input information of the implant position anddirection of the desired motion, and processes this information withpredetermined motion algorithms to effect a desired motion. In thismode, the forbidden regions (i.e., the regions including the body andthe imaging system that must be avoided by magnet system 12) areprovided as data to computer 32 or are determined by computer 32 in somesuitable manner. The computer 32 then, through software control,provides the desired alignment in a manner similar to that describedabove. Off-axis field lines of magnet 14 (such as lines 102 of a singlecoil magnet as shown in FIG. 4C) may be used to greatly increase theeffective directions of the magnet, and in fact provide completecoverage of the procedure region in patient 26 despite the forbiddenexclusion regions.

FIG. 5A is an illustration of a typical exclusion region in aneurosurgical procedure. The exclusion region 70, in which X-ray beamspass, is defined by two cones 72 and 74. The tips of the two cones 72,74 are separated by about 6″. The conical regions are slightly offsetfrom one another, so that near the patient's neck, the intersection isabout 2 inches lower than at the top of the head. This detail is bettershown in FIG. 5B, and is represented as dimension C. (For presentpurposes, it may be assumed that other exclusion regions exist,including one that encompasses the entire body 26. However, magnet 14and magnetic assembly 12 neither of which is shown in FIGS. 5A or 5B,may be provided with mechanical limitations that would prevent theirpositioning that far away from the patient's head 26A. Therefore, it isnot necessary to provide a control computer with data detailing theshape of exclusion regions beyond the limits of motion of magnet 12 andmagnetic assembly 14.) In FIGS. 5A and 5B, dimension A is typicallyabout 6 or 7 inches, and angles B are typically about 30°. Theillustrated exclusion zone 70 comprising cones 72, 74 provides ampleroom for manipulation of a magnet 14 having a shaped field on the left,right, and top of the head 26A, and perhaps some other areas, withoutinterfering with an imaging apparatus, parts of which are typicallylocated above and below the patient's head. The exclusion zone 70 shownhere represents the typical cumulative mechanical zone of exclusion andimaging beam zone of exclusion to accommodate 2″ X-ray equipment. FIG.5B better shows the typical maximal operating volume 26B of a magneticcatheter (not shown) with respect to exclusion zone 70.

The inventive apparatus may be used to perform magnetic surgery by amethod comprising the steps of implanting a magnetic seed in the body ofa patient, producing a medical image (such as an X-ray image) of theregion of the patient's body including the implanted seed, superimposinga representation of the magnetic field lines of an external magnet(i.e., magnet 14) on the medical image, aligning a portion of thesuperimposed representation, such as a near-field magnetic field line ofthe external magnet, along an intended path of the seed, and moving theexternal magnet into a position to produce the represented superimposedmagnetic field to effect guidance or movement of the seed. If the magnetis a permanent magnet, the alignment and movement steps would occursimultaneously. If the external magnet is an electromagnet, eithernormally conducting or superconducting, the alignment step may occurfirst, followed by servo-controlled movement of the magnet.

The inventive apparatus may also be used in a method comprising thesteps of implanting a magnetic seed in a patient's body; imaging aregion of the patient's body including the implanted seed to therebyproduce a medical image; storing information indicative of a path to betaken by the seed in a memory of a computer having a display; displayinga representation of the path and a current location of the seed on thedisplay; calculating orientations and currents of an externalelectromagnet required to provide a magnetic field to the seed to movethe seed along the path; activating a servo-controlled motor to move theexternal electromagnet to the calculated orientations; providing thecalculated currents to the electromagnet; and updating the display toprovide current locations of the seed relative to the path. Commands maybe provided to the computer by non-tactile means, such as by an operatorspeaking into a voice recognition unit that provides input to thecomputer.

One skilled in the art would recognize that numerous modifications arepossible to the specific embodiments described herein while remainingwithin the

What is claimed is:
 1. A method for providing a rapid interactivedisplay of at least one of the aligning torque and the magnetic pullingdirections of a magnet, the method comprising: (a) positioning a bodypart of a patient in a fixed position in space relative to plurality oflocalizers; (b) displaying a view of the body part within a field ofview of the localizers on a display; (c) identifying landmarks on thebody part; (d) locating fiducial marks on a moveable magnet assembly;(e) computing a position and orientation of the magnet assembly; (f)determining relationships between a coordinate system of the display,the magnetic assembly, and a stored representation of a magnetic fieldproduced by the magnet assembly; (g) converting the representation ofthe magnetic field into display coordinates; and (h) displaying arepresentation of the magnetic field in display coordinates on thedisplay.
 2. The method of claim 1 wherein the identification stepcomprises the touching of markers selected from the group consisting ofanatomical landmarks and fiducial markers with a localizing device.